Most gases are invisible to the unaided human eye, particularly at low concentrations. It is thus difficult, and sometimes impossible, to visually determine the presence and extent of releases of these gas into the environment. The ability to rapidly detect and track hazardous gases in the atmosphere would greatly aid public safety and health, and would be useful and in determining the source of gaseous leaks in general. For example, accidental toxic or combustible gas releases can occur from malfunctioning industrial equipment or from accidents involving the transport of bulk hazardous materials. These releases can rapidly diffuse into the surrounding air and move with the prevailing wind. While the safety of the public would be greatly enhanced in such circumstances by the easy determination of the location, extent, and motion of these gases, there is no device that is capable of providing this information.
The detection of leaks is of concern in industrial settings. For example, the natural gas and petroleum industries are mandated by law to regularly perform leak surveillance of their processing hardware and product pipelines. Existing detection technology is labor intensive and costly, requiring manually use of equipment that measures at a single point and in close proximity to the leak source. Leak inspections thus require approaching within 1 cm of tens of thousands of potential leak points per facility. In addition, point measurements of gas concentration do not provide information on the volume of release, and are of limited use in quantifying the amount of gas in a leak.
Backscatter absorption gas imaging (BAGI) is one advanced technique that shows promise for remotely producing real-time video images of otherwise invisible gases. A BAGI system consists of a light source that produces radiation that is absorbed by a gas of interest and a video camera that collects the light to produce images of the extent of the gas within an imaged scene. Specifically, light is directed to illuminate an area having a solid object (e.g., a wall) in the camera's field of view. The solid object scatters light back towards the camera, and if the gas of interest is present, it will absorb the backscattered light. Light that is thus backscattered is imaged, or processed to produce an image, of the scene that can be interpreted by the BAGI system user to determine the presence and position of gas in the environment. A BAGI image, for example, can consist of light and dark regions according to the amount of absorbing gases present. Brighter regions correspond to scenes having no, or small amounts of, absorbing gases, and darker regions correspond to scenes having higher amounts of absorbing gases. By adjusting the wavelength of the BAGI light source to correspond to the absorption of different gases, BAGI systems can produce images of the extent of these different gases.
The camera of a BAGI system thus produces an image of source light that is backscattered to the camera from solid surfaces in the scene of the camera field-of-view. As such, BAGI is limited to producing images of scenes containing a solid surface.
Prior art BAGI systems suffer from limitations that prevent them from being generally useful in producing real-time video images of gas in the environment. In particular, a useful BAGI system should have a light source that is 1) easily adjustable to provide light that is both transmitted through the atmosphere and absorbed by gases of interest, 2) has an output power high enough to enable measurements to be made at a distance, and 3) have low power consumption so the system can be portable. Prior art systems do not meet all of these requirements. In particular, no BAGI system exists that meets these requirements for imaging hydrocarbon gases. Another requirement for a useful BAGI system is compatibility with common and inexpensive cameras. This requirement is met with a light source that emits light compatible with scanning cameras. Pulsed format BAGI systems are not compatible with these cameras.
The lack of BAGI instrumentation that can address particular market needs has significantly impacted the size of the available market for BAGI instrumentation and has deprived certain industries of the benefits of the technology. For example, the petroleum industry is mandated to perform leak detection on a quarterly basis at each of their processing refineries. This is currently done using manually-positioned probes that must be placed in close proximity to thousands of potential leak points during a survey. A typical large refinery spends approximately $1,000,000 per year in leak detection and repair activities. The petroleum industry has recognized the potential of gas imaging as a means to perform these operations more rapidly. The American Petroleum Institute (API) recognizes gas imaging as a means to satisfy their goal of Smart Leak Detection and Repair (Smart LDAR). Prior to the invention described in this document, however, BAGI could not meet this need due to the lack of instrumentation capable of viewing hydrocarbon leaks, which are the primary emissions at a refinery. Similar unfulfilled needs exist in the natural gas industry, which must perform mandated leak detection operations on natural gas leaks in their pipelines and processing facilities. There, detection of natural gas (primarily methane) is required, which is again not possible with existing BAGI instrumentation. The need for a hydrocarbon-imaging BAGI system has existed for over fifteen years but remains unfulfilled.
The spectral requirements of a BAGI light source can be met with a spectrally-narrow and tunable light source that generates sufficiently powerful radiation in the infrared (IR). Specifically, the needs described in the previous paragraph can be met by illuminating with light in the wavelength range between 3 and 4 μm (frequencies between 2500 and 3333 cm−1) as some of these wavelengths correspond with spectroscopic features of hydrocarbon gases and are efficiently transmitted through the atmosphere.
Unfortunately, there is no commercially available, wavelength tunable infrared (IR) light source that meets all of the requirements for a BAGI system suitable for use in hydrocarbon leak detection and, thus, much of the work in developing BAGI systems has been directed to light source development. One light source that can potentially fulfill the needed requirements is the combination of a near-IR light source, such as a laser or diode with output at a wavelength of about 1 μm, that acts as a pump beam for a nonlinear frequency converter with an output in the 3 to 4 μm (2500 to 3333 cm−1) range. A range of output wavelengths results from tuning the light source and/or the frequency converter.
Currently available light sources using nonlinear frequency converters have limited utility as a BAGI light source. While these sources generate light of a useful wavelength, tunable near 3.3 μm, the power levels of 200–300 mW are insufficient to operate at distance greater than 2–4 m. In addition, these light sources suffer from other deficiencies that hinder their usefulness in portable devices. These limitations include: unstable light source behavior that varies from day-to-day, less than theoretical tuning range and power in practice, excessive light source cooling requirements, and difficulty in servicing the light source.
In summary, there are no known devices available either in development or in the marketplace that meet the requirements of a BAGI light source suitable for hydrocarbon detection in a useful way.
Therefore, it would be desirable to have a system that provides a portable gas imaging system, and thereby enables the use of gas imaging systems to sense the presence of leaks of hazardous or other visually transparent gases.